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I am more interested in its explanation, now that the theory has been proven correct again and again.
Especially interested in "delayed choice quantum erasure experiment", where you decide to determine the "which path" after the photon has passed through the slits and hit the detector. And depending on your later decision the photon seems to rewrite history going back in time.
I don’t have a source to hand at the moment, but when I looked into the famous Delayed Choice Quantum Erasure experiment the consensus seemed to be:
- The double slit experiment’s conclusions still hold, but:
- The particularly exciting and stark results of the Quantum Erasure experiment may have been misinterpreted or miscommunicated to the public, in particular:
- The presenter of PBS SpaceTime has said that he regrets certain things about how he worded his video on the Quantum Erasure experiment, and I think may have left a comment on the video to that effect.
Every time I look into QM, I keep coming back to the same fundamental axiom: “Quantum Mechanics’ weirdnesses can make otherwise straightforward things frustrating, but will never make interesting inventions possible.” Like how entanglement is able to break locality (which is frustrating) but without breaking causality (which would be interesting). If you hear about a quantum principle and think “Wow, I could use that to build X,” then it’s more likely that you’re not fully understanding the principle (not “you” specifically, I’ve fallen for this myself countless times).
The only exception seems to be Quantum Computing, but even that only arises out of a deep deep mathematical analysis (you can’t get to QC on your own from the things in popular science books) and is only applicable to really niche applications.
It seems fairly obvious that the universe avoids doing work when it’s not necessary. I agree we’ll probably be disappointed looking for magical ways to make it yield much more.
> The only exception seems to be Quantum Computing
Yet so far it failed to do any useful work, correct? As I understand it, even the recent "quantum supremacy" results were about performing a humongous number of useless computations.
It would be funny if this turned out to also apply to quantum computing. Ie while we can build a quantum computer, we can't actually find any productive problem that calculates faster than a classical algorithm.
QC would turn out to be the biggest bust in physics (after string theory of course).
So far yes, but we should be able to in principle for a few kinds of problems.
Entanglement doesn't violate locality, it's measurement that does that. And that's because we don't have a rigourous handle on what measurement actually is, and why we call it "the measurement problem"!
Didn't they originally use polarizing filters to measure photonic phase?
If it were possible to measure the phase of a photon after a beam splitter in a nondestructive way, shouldn't it be possible to determine whether measuring one causes state collapse in the other?
This says that photonic entanglement is polarization, and that photonic phase can be inferred from second order of intensity, IIUC:
"Bridging coherence optics and classical mechanics: A generic light polarization-entanglement complementary relation" (2023) https://journals.aps.org/prresearch/abstract/10.1103/PhysRev...
Shouldn't it then be possible to nondestructively measure photons and thus entanglement?
So you don't think a laser is an interesting invention? Those require quantum mechanics for the stimulated emission.
Quantum tunneling is key to many devices as well.
Then of course there's the reality that the mere existence of everything we see around us - the stability of atoms themselves - requires quantum mechanics.
>about a quantum principle and think “Wow, I could use that to build X,”
We use quantum principles to build things all the time. What are you talking about?
https://en.wikipedia.org/wiki/Quantum_sensor#Research_and_ap... is just a few examples.
I’m talking about building sexy things like ansibles or FTL engines. The kinds of transcendent ambitions that Quantum Mechanics often inspire in laypeople like me.
Every device one has that has integrated circuits (chips) and some of the discrete components was designed based on the quantum mechanical properties of materials that were beyond us until we understood quantum mechanics - sometimes we get too accustomed to the wonder right in front of us. The Electrical Engineering curriculum at my university started with Quantum Electrodynamics for a semester. Figuring out how the sun and stars work required an understanding of the smallest particles, it's all kind of amazing.
Even if we had a "stranglehold" on the deepest foundations of Quantum Mechanics it wouldn't help you with either of those things. In order for those things we need something beyond QM and a lot more grand. FTL travel and communication is in the ball park of relativity.
QM is not an umbrella term for sci-fi.
Quantum Enxryption!
Quantum physicist here. I can only say that reality down there at the quantum level is really really weird. You can get used to it, but forget making sense of it.
A delayed choice setup is not too dissimilar than a Bell inequality violation experiment. The weirdness there is that you can set things up such that no signal can travel between the systems being measured, and yet the outcomes are more correlated than any classical joint state can be.
So the conclusion is that either locality fails (i.e. it’s not true that outcomes on one side are independent of how you measure the other side) or realism fails (i.e. you can’t assign values to properties before the measurement, or in other words a measurement doesn’t merely “reveal” a pre-existing value: the values pop into existence in a coordinated fashion). Both of these options are crazy, and yet at least one of them must be true.
> either locality fails of realism fails
Or statistical independence fails, no? The CHSH derivation, for example, requires commuting expectation value with conjunction and similar for other Bell-like's that I'm aware of.
This always gets pooh-poohed away with with vague appeals to absurdism, "Alice and Bob's free will blah blah", but I don't really know of a priori reasons why the global state space needs to be Hilbert instead of a more complicated manifold with some Bell-induced metric. If you know of prior art here, I'd love some pointers.
That way lies superdeterminism, which has stood out to me as the most satisfactory explanation for years.
I’m not sure I understand. Expectation values are just scalars, why wouldn’t they commute? Can you explain what you mean?
To me, it is not hard to make sense of quantum reality and it’s not weird at all, it actually makes sense. And it makes sense to me because we are living in it.
If you’ve ever looked into the theory of Orch-OR I’m sure you’d understand what I’m talking about. The minute you think of the quantum being different from the classical is where the problems begin.
Classical physics is the only process we have to understand quantum physics. Our brains are quantum computers that collapse wave functions so we can navigate the universe. And by collapsing the wave function I just mean we make a probability the best certainty we can.
Light as a wave is a probability. Light as a particle is a certainty.
It's the measurement problem, I think? Energy is moving as a wave, but the energy can only be transferred in quantum-sized values. At some point it "collapses" to a particular interaction with some other wave, and we can only probabilistically calculate where this may occur.
Edit: the Bell experiment is something else. It's like a wave can exist as an entity outside of time and space and only comes back to reality when it interacts. Perhaps it would make sense for electromagnetic waves if the distance and local time elapsed contracts to zero per relativity when travelling at the speed of light.
The measurement problem is a different kind of weirdness, that may or may not reduce to the same explanation after we have it.
The problem with the double slit (and Bell inequality) is that real things that we can see are correlated, not about mixed states and state erasure.
My impression about reality is the opposite. The quantum world makes perfect sense while it's the emergence of the classical world which is unfathomable. The crazy "pop into existence" part is still incomprehensible, so I guess it's essentially the same.
https://philarchive.org/archive/ELLWDC
Why Delayed Choice Experiments do NOT imply Retrocausality
David Ellerman
University of California/Riverside
October 16, 2014
There is a fallacy that is often involved in the interpretation of quantum experiments involving a certain type of separation such as the: double-slit experiments, which-way interferometer experiments, polarization analyzer experiments, Stern-Gerlach experiments, and quantum eraser experiments. The fallacy leads not only to flawed textbook accounts of these experiments but to flawed inferences about retrocausality in the context of delayed choice versions of separation experiments.
My understanding is that this “temporal fuckery” (I’m not a physicist) exists even in the basic math of light diffraction. When light passes from air to water, it somehow “knows” the right angle to diffract at to reach its destination as fast as possible, even though from a classical viewpoint the destination is not known until after the light has passed through the medium.
The short story “Story of your life” (that the movie Arrival is based on) uses this as a pseudo-argument for how the aliens could have a non-temporal understanding of reality.
The standard explanation for light "knowing" the angle of diffraction is that actually light just propagates in every direction and then constructive interference is stronger for paths near the shortest path because its length is more consistent when the path is perturbed (meaning the phases of the perturbed paths tend to agree more so they add up instead of cancelling). I don't think you even need quantum mechanics for this; it occurs in classical wave optics.
You can see Feynman explaining mirrors this way in recorded lectures [1]. There's also a recent Veritaseum video explaining why the shortest paths dominate [2].
Thanks for sharing this. I was vaguely aware of this explanation – I believe I watched a Richard Feynman video where he describes this.
The funny thing is that while the standard quantum explanation for this behavior is different than the situation I described in my post, it’s somehow even less intuitive than believing that the light “knew“ where to go before it left it’s source. Instead we are supposed to assume that it checked all possible directions and instantaneously made a (“correct”) decision.
I am fully aware that my sentences above are full of anthropomorphisms and are hopelessly classical. But in my classically oriented mind, my first way of describing the situation and you’re more correct way both involve a violation of what I would consider “causality”.
There's another Veritasium video that has a neat experiment ostensibly showing light (both lamp and laser sources) "taking all paths" (or words to that effect--I don't really know what I'm seeing or what I'm talking about!) It starts around 25 minutes in.
https://www.youtube.com/watch?v=qJZ1Ez28C-A
EDIT: Whoops. The YouTube video linked by naasking in this comment: https://news.ycombinator.com/item?id=44771713 touches on the Veritasum video I linked to and goes to some length to explain that it is NOT proving the light is taking "all possible paths." He also brings up and links to a video on the "Looking Glass Universe" channel in which the hostess recreates the Vertiasium experiment and gives a differing interpretation. (Some commenters there have objections to the experimental setup. Oh boy, I may be down a rabbit hole here.)
That only works with quantum mechanics - it's a consequence of the "path integral" idea of QM. In classical optics this wouldn't work, because you'd be able to detect light on the other paths if it really did take all paths.
I don't think this "standard explanation" is as standard as it is sometimes portrayed:
>I think I can safely say that nobody understands quantum mechanics.
— Richard Feynman
> When light passes from air to water, it somehow “knows” the right angle to diffract at to reach its destination as fast as possible, even though from a classical viewpoint the destination is not known until after the light has passed through the medium.
That sentence brought to my mind all those “How does the mirror know what’s behind the paper?” videos.
> I am more interested in its explanation, now that the theory has been proven correct again and again.
What do you call an explanation? An interpretation of QM? There are dozens but none are especially satisfying..
As for the 'delayed choices' IMHO it is a poor interpretation of the data: see https://www.youtube.com/watch?v=RQv5CVELG3U for example.
https://youtu.be/fbzHNBT0nl0
This video blew my mind wide open about the double slit experiment by showing the simpler case, the single slit experiment, and I think it clears up a LOT! Sadly, I can't do the explanation any justice
My understanding is that a lot of the weirdness is that there may be no such thing as a photon, that is, what we call a photon is the em field interacting with matter(electrons really), in transit there is no such thing as a "single photon" That is, the em field is not quantized( or at least not quantized at the level of a photon) It is very possible to have your em field at "sub photon level" and the electrons at the far end will accumulate energy until a photon level is reached and presto a single photon experiment. What it is actually measuring I am not sure.
https://www.youtube.com/watch?v=SDtAh9IwG-I (Huygens Optics -How big is a visible photon?)
My understanding of the delayed choice experiment is that causality is preserved only because you can't actually determine the result of a single event without comparing the two results. It's spooky action at a distance, except it's temporal instead of spatial distance.
Which honestly I find to be pretty flimsy reasoning. It's almost ontological: causality is preserved because we can't prove that it isn't.
I know it's more complex than that, but it still feels like papering over a hole in our theories.
You might want to watch Hossenfelder’s video on the subject.
Because you are definitely not rewriting history, and absolutely nothing is traveling back in time whatsoever.
My layman's guess is that our interpretation of the fundamental explanation for quantum mechanics is wrong. That wave-particle duality is wrong. There is an old alternative explanation which has been gaining some attention lately: pilot wave theory. The TL;DR is that there is both a wave and particle. The particle generates the wave, but is also influenced by interacting with it. Veritassiun has a great video on it which is compelling.
But again, I am not a physicist. Just an enthusiastic outside observer.
> The particle generates the wave, but is also influenced by interacting with it.
Oh wow. So the particle and wave are like a planet and its gravity (in a sort of loosely analogous way)?
You are correct that the pilot wave theory (Bohmian mechanics) says that instead of wave OR particle, it is wave AND particle.
But the particle does not generate the wave. There is one wave function governing the whole universe. It is a function on the 3n-dimensional configuration space of all of the particle positions. To find the velocity of a given particle at a given time, one needs to put in the position of all of the particles of the universe. Practically speaking, in an experimental setup, the macro state of the environment is sufficient to create an effective wave function of the particle which is how we can effectively use quantum mechanics on a subsystem of the universe. The collapse of the wave function in measurements is a reflection that once the little system interacts enough with the environment, then the separate environmental configurations have separated out the behavior of the wave relative to the one particle so that an effective collapse wave function can be used.
This plugging in the configuration of all the particles is a gross violation of a relativistic outlook (what is the universal now?). Bell after seeing Bohm's theory immediately grasped the implications and wanted to know if that nonlocality could be removed. His work, along with EPR, was to demonstrate that there was no theory of any kind that could avoid the non locality if results of experiments actually happen when we think they do.
The double slit experiment is perfectly explained by the approximate wave function of the 1 particle system going through both slits and interfering with itself while the particle is guided by that wave which is why there is an interference pattern that builds up out of particular particle dots. There is nothing other than practical difficulties to make the wave separation happen later but have outcomes as if it didn't; it is all about what the wave function is doing as the particle is most likely to be where |psi|^2 dictates it to be. That is what the law of motion assures. One could theoretically simulate the paths conforming to make this happen though the paths themselves could have quite unexpected behavior.
There are various extensions to Bohmian mechanics to deal with particle creation, annihiliation, quantum field theory, and relativistic versions. None are as complete as non-relativistic quantum mechanics in having a mathematically proven existence, but a large part of that is quantum field theory being unsound; the Bohmian part is not a problem. There are avenues being pursued to solve the quantum field theory infinite divergences using Bohmian insights (basically use wave functions that respect probability flowing along with particle creation and annihilation). The work is promising but difficult.
For the relativistic versions, it is easy enough to create a foliation of space-time to create a "now". There are even versions where the foliation is created out of what is already existing structure. Mathematically it seems fine as far as I know. But philosophically, it is weird to have an invisible fundamental structure existing that seemingly contradicts the main lesson of relativity.
Maybe someone in the field can speak up -- I'm not sure what is new about this study. It seems to be about an analysis of the double-slit experiment using individual atoms, and the press release implies that this is novel, but that experiment was first done over 30 years ago [0]. Is there anything to this study that is actually new?
0: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.70...
It seems that photons propagate as massless non-local waves, but become localized when they bounce off an already local, massy particle. In other words, electromagnetic radiation is a fundamentally continuous phenomenon in a continuous field, but emission, absorption and other interactions can only happen as discrete events (quanta).
Can anyone explain what sort of "detector" is used to detect the photons? Is there a standard across all double-slit experiments?
I feel like the type of detector would have a significant impact on the outcome of the experiment, but explanations for layman always leave it out.
Quantum Interference 1: A Simple Example:
https://profmattstrassler.com/2025/03/18/quantum-interferenc...
I don't get the point. The article says that if you "somewhat" measure, then you lose "somewhat" from the wavelike nature. So the photon is a wave by X%, and a particle by 100-X%?
A quantum object is its own thing - it has both wavelike and particle-like properties.
Measurement here might be better understood to "filter out" any parts of the wave that don't agree with the measurement. So a precise measurement will project out a lot of the wave, giving you something more localized and particle-like. A fuzzy measurement will project out only a bit of the wave, giving you something that's still spread out and quantum and wave-like.
The article says "The fuzzier atom rustles more easily and records the path of the photon. In tuning up an atom’s fuzziness, researchers can increase the probability that a photon will exhibit particle-like behavior".
I think we're just seeing decoherence in action here. If the photon interacts with the atom, it becomes entangled with the environment (the atom). Giving the atom a higher temperature results in it having a higher probability of it interacting with the photon, and decohering.
And I think the individual photon doesn't have a mixture of a certain % of wave or particle like nature. It's just that there is a certain probability that it will decohere (interact with the atom), so if you turn up the temperature of the atoms, you'll just see a greater % of the photons decohering when they interact with those atoms.
That's just my amateur understanding of the situation, so I'm happy to be corrected by someone who knows better. Also, I don't have access to the paper itself (https://journals.aps.org/prl/abstract/10.1103/zwhd-1k2t) as it's paywalled and not on scihub.
Quantum mechanics is fascinating!
This is the biggest misunderstanding. Light is always a wave. It is never a proton. Light becomes a proton when we measure it. Everything is a wave, and nothing is a particle ever.
Waves are just probabilities and the human quantum computer brain collapses those probabilities in an orchestrated reductive capacity to create a certainty out of a probability.
The simulation rolls back to match the constraints, easy